1. Field of the Invention
The present invention relates to an electronic apparatus and particularly a technique for protecting a substrate in an operation portion from an electrostatic discharge.
2. Description of the Related Art
Conventionally, an operation portion of an electronic apparatus has adopted a structure which covers a metal dome with an insulating sheet to protect the operation portion from an electrostatic discharge. For example, a technique for covering a metal dome with an insulating sheet in order to protect an electronic circuit board from an electrostatic discharge is discussed in Japanese Patent Application Laid-Open No. 2001-513941.
However, in the conventional technique, when static electricity enters inside from a gap between an exterior cover and an operation button, the static electricity is discharged to an electronic component on the electronic circuit board. Thus, protection of the electronic circuit board is not sufficient.
Further, if the operation button is configured by a conductive member, the conductive member is electrically independent. Thus, a charge of the static electricity is accumulated on the operation button, so that a secondary discharge of the static electricity may be generated. Consequently, an electrostatic discharge may be generated to electronic component or the like on the electronic circuit board. Thus, the conventional technique has been lacking for protection of the electronic circuit board.
FIG. 10A illustrates a partial cross sectional view around an operation button in a conventional operation portion. Further, an enlarged view around a contact point of the operation button is illustrated in FIG. 10B.
In FIG. 10A, the operation portion includes a cover member 101 as an exterior cover, an operation button 108, and a pusher 121 of the operation button 108. A metal dome 131 and an insulating sheet 132 form a metal dome sheet. The operation portion further includes an electronic circuit board 140, an electronic component 141 such as a light emitting diode (LED) disposed on the electronic circuit board, a switch detecting pattern 142 on the electronic circuit board, and a ground pattern 143 on the electronic circuit board.
An adhesive material 130 is used for bonding the metal dome sheet to the electronic circuit board 140, and includes, for example a double-sided tape or the like. The adhesive material 130 is provided with one or more holes so that the metal dome 131 is brought into contact with the electronic circuit board 140 when the metal dome 131 is pressed by the pusher.
The metal dome 131 is covered with the insulating sheet 132. When static electricity is discharged, as a path 150a in FIG. 10A, the static electricity enters inside from a gap between the cover member 101 and the operation button 108. Although the metal dome 131 is covered with the insulating sheet 132 and is protected from the static electricity, the electronic component 141 is exposed on the electronic circuit board 140 and the static electricity travels in the air as illustrated by the path 150a and is discharged. Thus, the static electricity will travel to the electronic circuit board 140 via the electronic component 141 and affect them. Therefore, in this case, it is necessary to provide an electric grounding by another conductive material to prevent occurrence of a discharge or a secondary discharge of the static electricity.